It is well known to those skilled in the art that polyisocyanurate foams can be prepared by reacting and foaming a mixture of ingredients, consisting in general of an organic polyisocyanate (including diisocyanate) and an appropriate amount of polyol or mixture of polyols in the presence of a volatile liquid blowing agent, which is caused to vaporize by the heat liberated during the reaction of polyisocyanate and polyol. It is also well known that this reaction and foaming process can be enhanced through use of amine and metal carboxylate catalysts as well as surfactants. The catalysts ensure adequate curing of the foam while the surfactants regulate and control cell size.
In the class of foams known as low density rigid polyisocyanurate foam, the blowing agent of choice has been trichlorofluoromethane, CCl.sub.3 F (known in the art as CFC-11). These types of foams are closed-cell foams in which the CFC-11 vapor is encapsulated or trapped in the matrix of closed cells. They offer excellent thermal insulation, due in part to the very low thermal conductivity of CFC-11 vapor, and are used widely in insulation applications, e.g. roofing systems and building panels. Generally, 1 to 60 parts by weight and typically, 15 to 40 parts by weight of blowing agent per 100 parts by weight polyol are used in rigid polyisocyanurate formulations.
In 1985, about 140 MM lbs. of blowing agents including CFC-11 and dichlorodifluoromethane (known in the art as CFC-12) were used in the U.S. to produce all types of insulation foams. Of this total volume, about 70% or 100 MM lbs. were used to make polyurethane foam. Closed-cell polyurethane foam is the most energy efficient insulating material available having an R value of approximately 7.2 per inch whereas fiberglass has an R value of approximately 3.1 per inch.
Closed-cell polyurethane foams are widely used for insulation purposes in building construction and in the manufacture of energy efficient electrical appliances. In the construction industry, polyurethane modified polyisocyanurate board stock is used in roofing and siding for its insulation and load-carrying capabilities. Poured and sprayed polyurethane foams are also used in construction. Sprayed polyurethane foams are widely used for insulating large structures such as storage tanks, etc. Pour-in-place polyurethane foams are used, for example, in appliances such as refrigerators and freezers plus they are used in making refrigerated trucks and rail cars.
In the early 1970's, concern began to be expressed that the stratospheric ozone layer (which provides protection against penetration of the earth's atmosphere by ultraviolet radiation) was being depleted by chlorine atoms introduced to the atmosphere from the release of fully halogenated chlorofluorocarbons. These chlorofluorocarbons are widely used as propellants in aerosols, as blowing agents for foams, as refrigerants, and as cleaning/drying solvent systems. Because of the great chemical stability of fully halogenated chlorofluorocarbons, according to the ozone depletion theory, these compounds do not decompose in the earth's lower atmosphere but reach the stratosphere where they slowly degrade liberating chlorine atoms which in turn react with the ozone.
During the period of 1978 to the present, much research was conducted to study the ozone depletion theory. Because of the complexity of atmospheric chemistry, many questions relating to this theory remain unanswered. However, if the theory is valid, the health risks which would result from depletion of the ozone layer are significant. This, coupled with the fact that world-wide production of chlorofluorocarbons has increased, has resulted in international efforts to reduce chlorofluorocarbon use. Most recently, the United States Clean Air Act calls for total phaseout of CFC's by the year 2000.
Because of this proposed reduction in availability of fully halogenated chlorofluorocarbons such as CFC-11 and CFC-12, alternative, more environmentally acceptable products are urgently needed.
As early as the 1970's with the initial emergence of the ozone depletion theory, it was known that the introduction of hydrogen into previously fully halogenated chlorofluorocarbons markedly reduced the chemical stability of these compounds. Hence, these now destabilized compounds would be expected to degrade in the lower atmosphere and not reach the stratosphere and the ozone layer. Table I lists the ozone depletion potentials for a variety of fully and partially halogenated halocarbons. Greenhouse potential data (potential for reflecting infrared radiation (heat) back to earth and thereby raising the earth's surface temperature) are also shown. In Table I, the ozone depletion potentials and greenhouse potentials were calculated relative to CFC-11.
TABLE I ______________________________________ Ozone Depletion Greenhouse Blowing Agent Potential Potential ______________________________________ CFC-11 (CFCl.sub.3) 1.0 1.0 CFC-12 (CF.sub.2 Cl.sub.2) 1.0 3.1 HCFC-22 (CHF.sub.2 Cl) 0.05 0.36 HCFC-123 (CF.sub.3 CHCl.sub.2) 0.015 0.02 HCFC-141b (CFCl.sub.2 CH.sub.3) 0.15 0.15 ______________________________________
Halocarbons such as HCFC-123 and HCFC-141b are environmentally acceptable in that they theoretically have minimal effect on ozone depletion.
Organic substances which bear a hydrogen on one carbon and a halogen (F, Cl, Br, I) on an adjacent carbon, will undergo so-called elimination reactions, under the influence of bases or acids, to produce haloalkenes and hydrogen halides or products from the combination of the hydrogen halide with the base, known as salts.
Therefore, in view of the fact that some of the major and many of the minor components, i.e., polyols and catalysts (amines, metal salts), are of known basic character, dehydrohalogenation of the above mentioned hydrohalocarbons might occur. Examples of such reactions are as follows: ##STR1##
Many of these haloalkenes possess unknown properties and it is therefore desirable to hold their formation to a minimum as a precautionary measure.
Tests performed using the above hydrohalocarbons as blowing agents, in typical foam formulations now in commercial use, revealed that the haloalkenes can be found in the cells of the cured foam at concentrations as high as 10,000 parts per million/weight relative to the blowing agent.
Stabilizers have been added to hydrohalocarbons to inhibit or minimize the generation and buildup of degradation products. For example, Kokai Patent Publication 103,843 published May 22, 1986 teaches that the addition of benzotriazole stabilizes 1,2-dichloro-1-fluoroethane when it is exposed to metallic surfaces in the presence of hydroxylic solvents, e.g. water or alcohols. Kokai Patent Publication 132,539 published May 25, 1989 teaches the addition of nitro compounds, phenols, amines, ethers, amylenes, esters, organic phosphites, epoxides, and triazoles to 1,2-dichloro-1-fluoroethane containing compositions in order to stabilize the compositions upon contact with metallic cleaning apparatus. Kokai Patent Publication 139,539 published Jun. 1, 1989 teaches the addition of nitro compounds, phenols, amines, ethers, amylenes, esters, organic phosphites, epoxides, furans, alcohols, ketones, and triazoles to 1,2-dichloro-1,1,2-trifluoroethane containing compositions in order to stabilize the compositions upon contact with metallic cleaning apparatus. U.S. Pat. No. 4,861,926 teaches that 1,1,1-trichloroethane can be stabilized with mixtures of epoxybutane, nitromethanes, 2-methylfuran, and methyl acetate in textile dry cleaning and metal degreasing applications. The Abstract of Japanese 2,204,424 published Aug. 14, 1990 teaches that hydrochlorofluoropropanes in the presence of steel are thermally stabilized by adding nitro compounds, phenols, amines, ethers, esters, epoxides, alcohols, ketones, or triazoles.
Specialized chemical additives are often present in low density rigid polyurethane and polyisocyanurate foams to enhance certain performance features of the foam e.g. flame retardants, antioxidants, and solubilizing surfactants. Such additives are dissolved in a formulation component or pre-mix prior to foam production. Flame retardants include halocarbons, e.g. chloroalkyl phosphate esters, polybromoalkanes, or polybromoaromatics. Antioxidants are typically phosphite esters. Solubilizing agents commonly used are ethoxylated nonylphenols.